Re: [RFC PATCH v5 1/4] topology: Represent clusters of CPUs within a die

[Greg KH]

On Fri, Mar 19, 2021 at 09:36:16AM +0000, Jonathan Cameron wrote:
> On Fri, 19 Mar 2021 06:57:08 +0000
> "Song Bao Hua (Barry Song)" <song.bao.hua@xxxxxxxxxxxxx> wrote:
> 
> > > -----Original Message-----
> > > From: Greg KH [mailto:gregkh@xxxxxxxxxxxxxxxxxxx]
> > > Sent: Friday, March 19, 2021 7:35 PM
> > > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> > > Cc: tim.c.chen@xxxxxxxxxxxxxxx; catalin.marinas@xxxxxxx; will@xxxxxxxxxx;
> > > rjw@xxxxxxxxxxxxx; vincent.guittot@xxxxxxxxxx; bp@xxxxxxxxx;
> > > tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; lenb@xxxxxxxxxx; peterz@xxxxxxxxxxxxx;
> > > dietmar.eggemann@xxxxxxx; rostedt@xxxxxxxxxxx; bsegall@xxxxxxxxxx;
> > > mgorman@xxxxxxx; msys.mizuma@xxxxxxxxx; valentin.schneider@xxxxxxx; Jonathan
> > > Cameron <jonathan.cameron@xxxxxxxxxx>; juri.lelli@xxxxxxxxxx;
> > > mark.rutland@xxxxxxx; sudeep.holla@xxxxxxx; aubrey.li@xxxxxxxxxxxxxxx;
> > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > linux-acpi@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; xuwei (O) <xuwei5@xxxxxxxxxx>;
> > > Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>; guodong.xu@xxxxxxxxxx; yangyicong
> > > <yangyicong@xxxxxxxxxx>; Liguozhu (Kenneth) <liguozhu@xxxxxxxxxxxxx>;
> > > linuxarm@xxxxxxxxxxxxx; hpa@xxxxxxxxx
> > > Subject: Re: [RFC PATCH v5 1/4] topology: Represent clusters of CPUs within
> > > a die
> > > 
> > > On Fri, Mar 19, 2021 at 05:16:15PM +1300, Barry Song wrote:  
> > > > diff --git a/Documentation/admin-guide/cputopology.rst  
> > > b/Documentation/admin-guide/cputopology.rst  
> > > > index b90dafc..f9d3745 100644
> > > > --- a/Documentation/admin-guide/cputopology.rst
> > > > +++ b/Documentation/admin-guide/cputopology.rst
> > > > @@ -24,6 +24,12 @@ core_id:
> > > >  	identifier (rather than the kernel's).  The actual value is
> > > >  	architecture and platform dependent.
> > > >
> > > > +cluster_id:
> > > > +
> > > > +	the Cluster ID of cpuX.  Typically it is the hardware platform's
> > > > +	identifier (rather than the kernel's).  The actual value is
> > > > +	architecture and platform dependent.
> > > > +
> > > >  book_id:
> > > >
> > > >  	the book ID of cpuX. Typically it is the hardware platform's
> > > > @@ -56,6 +62,14 @@ package_cpus_list:
> > > >  	human-readable list of CPUs sharing the same physical_package_id.
> > > >  	(deprecated name: "core_siblings_list")
> > > >
> > > > +cluster_cpus:
> > > > +
> > > > +	internal kernel map of CPUs within the same cluster.
> > > > +
> > > > +cluster_cpus_list:
> > > > +
> > > > +	human-readable list of CPUs within the same cluster.
> > > > +
> > > >  die_cpus:
> > > >
> > > >  	internal kernel map of CPUs within the same die.  
> > > 
> > > Why are these sysfs files in this file, and not in a Documentation/ABI/
> > > file which can be correctly parsed and shown to userspace?  
> > 
> > Well. Those ABIs have been there for much a long time. It is like:
> > 
> > [root@ceph1 topology]# ls
> > core_id  core_siblings  core_siblings_list  physical_package_id thread_siblings  thread_siblings_list
> > [root@ceph1 topology]# pwd
> > /sys/devices/system/cpu/cpu100/topology
> > [root@ceph1 topology]# cat core_siblings_list
> > 64-127
> > [root@ceph1 topology]#
> > 
> > > 
> > > Any chance you can fix that up here as well?  
> > 
> > Yes. we will send a separate patch to address this, which won't
> > be in this patchset. This patchset will base on that one.
> > 
> > > 
> > > Also note that "list" is not something that goes in sysfs, sysfs is "one
> > > value per file", and a list is not "one value".  How do you prevent
> > > overflowing the buffer of the sysfs file if you have a "list"?
> > >   
> > 
> > At a glance, the list is using "-" rather than a real list
> > [root@ceph1 topology]# cat core_siblings_list
> > 64-127
> > 
> > Anyway, I will take a look if it has any chance to overflow.
> 
> It could in theory be alternate CPUs as comma separated list.
> So it's would get interesting around 500-1000 cpus (guessing).
> 
> Hopefully no one has that crazy a cpu numbering scheme but it's possible
> (note that cluster is fine for this, but I guess it might eventually
> happen for core-siblings list (cpus within a package).
> 
> Shouldn't crash or anything like that but might terminate early.

We have a broken sysfs api already for listing LED numbers that has had
to be worked around in the past, please do not create a new one with
that same problem, we should learn from them :)

thanks,

greg k-h